Laser initiated three electrode type triggered vacuum gap device

ABSTRACT

A three electrode triggered vacuum gap device with the triggering mechanism being laser initiated. The triggering mechanism comprises a gas-saturated portion, which when heated by a triggering laser beam releases a gas. The gas released by the triggering mechanism is then ionized by an electric field, existing between the trigger electrode and the main electrode, starting a low power arc. The low power arc is then magnetically forced between the main arcing electrodes to initiate a main power arc. The laser-initated trigger provides electrical isolation between the vacuum gap device and the triggering control circuit.

United States Patent 11 91 Voshall LASER INITIATED THREE ELECTRODE TYPE TRIGGERED VACUUM GAP DEVICE [75] Inventor: Roy E. Voshall, New Alexandria,

[73] Assignee: Westinghouse Electric Corporation, I

Pittsburgh, Pa. [22] Filed Oct. 25,1972

[2]] Appl. No.: 300,616

[52] US. Cl 315/150, 3l3/l78, 315/36,

315/156 51 1111.010... H05b 41/04 58 Field of Search 3l5/36,'l49, 150, 156,

[56] References Cited UNITED STATES PATENTS 3517.256 6/1970 Barbini ..3l5/l56X 1 11 3,811,070 1451 May 14, 1974 Primary Examiner-Herman Karl Saalbach Asxistanl Examiner-James B. Mullins Attorney, Agent, 0r Firm-H. G. Massung 57 1 Y ABSTRACT 1 IA three electrode triggered vacuum gap device with 'the triggering mechanism being laser initiated. The

triggering mechanism comprises a gas-saturated portion, which when heated by a triggering laser beam releases a gas. The gas released by the triggering mechanism is then ionized by an electric field, existing between the trigger electrode and the main electrode, starting a low power arc. The low power arc is then magnetically forced between the main arcing elec-' .trodes to initiate a main power are. The laser-initated trigger provides electrical isolation between the vacuum gap device and the triggering control circuit.

' 11 Claims, 6 Drawing Figures LASER 'INI EIA DED THREE :EIJECTRQDE TYPE TR IGG-ERED VACUUM GAP DEVICE BACKGROUND OF THE INVENTION such as its ability to withstand high voltages with .a

small electrode separation and its quick recovery of its dielectric strength-afterarching. These desirable characteristicsjhavemade the vacuum gap device attractive for an overvoltage protection device .and .a current switch. The use of the vacuum gap to limit voltage surges on transmission lines is well knowninthe art, as discussed by J .M. Lafferty in his article Triggered VacuumGaps", Proceedings IEEE, Volume '54, No. ll,- January 196.6, page .23. Triggered vacuum gaps have 'a number of applications, such as high-speed circuitzprotection devices with high-voltage power transmission lines. Other applications include lightning arresters,

over-voltage protection for series capacitorsand-parallel operation with switches and breakers. I

It is not feasible to control the breakdown voltage of a vacuum gap device accurately by adjustment of the electrode spacing. It is necessary to provide an aux'ili- I ary trigger device to break down the vacuum gapwhen anexternal signal'is applied. The triggering mechanism must break down the vaccum gap quickly with amin imumof jitter. Ithas been found thattriggerin-g breakdown of the gap by injecting a plasma into the main I vacuum gap gives the most rapid breakdown with a minimum amount-of voltagejitter. As discussed in US. Pat. No. 3,538,382 by Sidney R. Smith, Jr..=issued Nov. 3, 1970, one desirable way ofinjecting:plasmainto the .2 internal of one of the main electrodes, and is electrically insulated from the associated main electrode. A

.portion of the triggering electrode or the associated main-electrode comprises a material that is saturated with a gas, such as hydrogen, that is rapidly released when the gas saturated portion is heated. It is to be unmain vacuum gap is to provide anauxiliary gas'loaded triggering electrode thatevolvesaplasma when-energized. However, the prior 'art'triggered vacuum gapdevice, as disclosed in US. Pat. No. 3,538,382, requires a separate triggering andpower supply circuits, at line potential, to operate the trigger. These circuits may have to be well insulated from ground potential,.and

this is especially true when the'triggered gap is connected in series with-one phase of a three-phase power line.

A method otbreaking down the vacuum gapthat has been found to be very effectiveand to give satisfactory performanceis the use of gas-loaded'electrodes. Hydrogen is the gas normally useddue to its ease of'loading and the rapidity with which it.is released fromthe gas loaded electrode on heating. Only minute quantities of hydrogen are released with no resultant buildaup of hydrogen pressure after repeated operation of the triggered vacuum gap. The plasmaproducedby the triggeringelectrode may be drivenatqahigh velocity into the main-vacuum gap, by the resultant magnetic field,'to initiate apower arcbetween the main elec-.

trode.

SUMMARY OF THE INVENTION According to the presentinvention a laser initiated threeelectrode type triggered vacuum gap device is provided. The triggered .vacuum'gap device comprises a pair of main arching electrodes within-an insulating vacuum housing. The triggering electrode is positioned derstood that the gas saturated material can be a separate piece attached to the electrode or an integral portion of the electrode. A voltage potential is applied between the trigger electrode and its associated main electrode. The applied voltage is lower than the voltage required to cause a breakdown between the trigger electrode and-theassociated main electrode. To initiate a high-power am, a laser beam is projected onto the gas-saturated material through a passage in the opposite main arcing electrode. This procedure liberates gas from .the gas-saturated material into the discharge region, between the triggering electrode and the associated main electrode, producing a low-power arc. The low power arc forms quickly between the trigger electrode and the associated main electrode. When the disclosed device is operated on an alternating current :power line having an operating frequency of 50 Hz or 60 H-z, the time duration of the laser beam is very short compared to the period of the power frequency. The main electrode, within which the trigger electrode is contained,is constructed so that a current loop flows through the arc and the trigger electrode and the associatedmain electrode to cause a magnetic force on the arc. The resultant magnetic force rapidly drives the arc into the interelectrode region of the main arching electrodes. The introduction of the low-power are into the main interelectrode region initiates a power arc across the main arcing electrodes.

When initiating breakdown by a pulsed laser beam being directed onto the gas-saturated material, the energyof this laser beam must be sufficient to heat the gas-saturated material to the point to cause some of the absorbed gas to be quickly liberated from it.

In one embodiment of the invention the laser is directed onto the triggering electrode which comprises a portion saturated with a gas. The'trigger electrode is disposed inside of an opening in the associated main electrode. The end portion of the trigger electrode, projecting inside the associated main electrode, is surrounded by an insulating piece that is smaller in diameterthan' the inner diameter of the opening through the associated main electrode. The trigger electrode is also recessed from the primary arching surface of the associated main arcing electrode.- By this arrangement metalvapors and particles which are dispersed from the main electrode during power arcing are less likelyto be deposited on the walls of the insulating material causing a shortening path to exist between the triggering electrode and the associated main electrode.

In another embodiment, the gas-saturated material is attached to'the main electrode wall which has been beveled so that the laser beam canbe focused on the gas-saturated material, rather than on the triggering electrode, in order to initiate the low-power arc.

In yet another embodiment, a gas-saturated metal discis attached to the trigger electrodeand a portion of the trigger electrode extends through the gassaturated metal disc. The gas-saturated metal disc and thertrigger electrodes are electrically insulated from the associated main electrode by vacuum and a solid high dielectric material. A portion of the solid insulating material between the metal disc and the associated main electrode 'is undercut to lessen the possibility of this area being coated with arc-generated metallic products and shorting the triggering electrode to the associated main electrode. A low power laser beam can then be directed onto the gas-saturated metal disc to initiate a low-power are, which in turn will cause a high power are to form between the main arcing electrodes.

The invention disclosed in this specification differs from prior art three electrode triggered vacuum gap devices, as shown in US. Pat. No. 3,538,382, in that the low-power are between the triggering electrode and the associated main electrode is laser initiated. The initiating laser and the related triggering control circuitry is electrically isolated from the vacuum gap device and can be located at a ground potential. Having the triggering control circuitry at ground potential is a desirable safety feature, and permits easy checking and maintenance of the control circuit while the vacuum gap device is in service.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention reference may be had to the preferred embodiment, exemplary of the invention, shown in the accompanying drawings in which: FIG. I is a sectional view of a laser-initiated threeelectrode type triggered vacuum gap device with a DC potential applied between the trigger electrode and the associated main electrode;

FIG. 2 is a graphic representation of the breakdown voltage and the applied DC voltage between the trigger electrode andthe associated main electrode;

FIG. 3 is similar to FIG. 1 but showing an alternating current voltage applied between the trigger electrode and the associated main electrode; 6

FIG. 4 is a representation of the voltage applied and the breakdown voltage between the-trigger electrode and the associated main electrode;

FIG. 5 is a view of another embodiment of the invention showing a portion of the main electrode through which the triggering electrode passes; and FIG. 6 is a view similar to FIG. 5 showing another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now tothe drawings and FIGS. 1 and 3 in particular, there is shown a three-electrode triggered vacuum gap device 10. The vacuum gap device 10 comprises a highly evacuated tubular envelope 12' formed from glass-of suitable ceramic material and a pair of metallic end caps 14 and 16 closing offthe ends of the insulating envelope l2. Suitable seals 18 are provided between the end caps 14 and 16 and the insulating envelope 12 to render the inside of the insulating envelope l2 vacuum tight. The vacuum in the envelope 12 under normal operating conditions is lower than 10 torr so that .the mean free path of electrons will be longer than the potential breakdown distance within trode 20 is rigidly secured to a conducting metal tube 26 by a suitable means such as welding or brazing. The conducting tube 26 is secured at its upper end to the triggering end cap 14. A triggering electrode 24 extends internal of the hollow conducting tube 26. Attached to the end of the trigger electrode 24 which is disposed within the conducting tube 26 is a portion 28 made from a gas-saturated material. The trigger electrode 24 is insulated from the main electrode 20 and the conducting tube 26 by a solid insulating member 30. A portion 32 of the insulating member near the gas-saturated piece 28 is ofa smaller diameter than the inner diameter of the conducting tube 26. The portion of the insulating member 30 in contact with the conducting tube 26 provides a suitable vacuum seal within the conducting tube 26.

The lower arcing contact 22 is joined to a conducting tube 34. The conducting tube 34 is rigidly attached to the laser end cap 16. Disposed within the conducting tube 34 is a transparent window member 36. Member I 36 provides a suitable .vacuum seal for the center of conducting tube 34. The transparent window member 36 is recessed axially from the end of the conducting tube 34 to which contact 22 is attached.

During operation of the vacuum gap device 10, a 'metallic are 38 is initiated between the separated electrodes 20 and 22 and serves as a vehicle for current conduction until the arc is extinguished. In an alternating current circuit the are 38 is usually extinguished near the first current zero of the alternating current wave. The are 38 that is established between electrodes 20 and 22 vaporizes and melts some of the electrode material. The vapors and particles are dispersed from the interelectrode arcing area 40 towards the inside of the insulating envelope 12. The internal surfaces of the insulating envelope 12 are protected from condensation of the arc-generated metallic vapors and particles thereon by means of a tubular metallic shield 42. The tubular metallic shield 42 is supported on the insulating envelope l2 and preferably electrically isolated from both end caps 14 and 16. This shield 42 actsto intercept and condense arcgenerated metallic vapors before they can reach the insulating envelope 12. To further reduce the chances of metallic vapors or particles bypassing the main shield 42 of a pair of end shields 44 and 46 are provided at the opposite ends of the main central shield 42.

The window member 35 is recessed in the conducting tube 34, toward the laser end cap 16, a sufficient distance so that arc-generated metallic vapors and particles cannot easily condense on the surface of the window 36, which is exposed to the internal environment of vacuum interrupter 10. The upper portion 32 of the insulating member 30 is of a smaller diameter than the conducting tube 26; so that, arc-generated metallic vapors and particles cannot readily condense on the insu- Iating member 30, so as to cause a short circuit current path between the trigger electrode 24 and the associated main electrode 20.

In operation of the triggered vacuum gap 10, a voltage is applied across the trigger electrode 24 and conducting stem 26. The voltage applied between electrode 24 and stem 26 can be either direct current (DC) as shown in FIG. 1 or alternating current (AC) as shown in FIG. 3. A graphic representation of voltage applied between the trigger electrode 24 and the associated main electrode 20 is shown for a DC voltage level 70 in FIG. 2, and the maximum value of an AC voltage level72 in FIG. 4. However, the applied voltage maximum 70 or 72 'must be less than the vacuum breakdown value 74, but large enough to initiate a lowpower are when plasma is introduced into the interelectrode area between electrodes 20 and 24. As shown in FIG. 1, voltage applied between members 24 and 26 can be DC and can come from a DC supply indicated as V1. A trigger circuit limitingresistor R] is placed in the circuit to limit the current during arcing. As shown -in FIG. 2, the voltage output of V1, indicated by numeral 70, must be less than the breakdown level 74 between the trigger electrode 24 and the conducting tube 26. As shown in FIG. 3, the voltage applied between trigger electrode 24 and conducting stem 26 may be alternating. A step-down transformer T1 steps the main circuit voltage down to a suitable level or operation of i the triggering mechanism. A voltage clipper 48 is provided to limit the voltage-VT applied berween the trigger 24 and stem 26 to a voltage level 72 less than the breakdown level 74. As can be seen in FIG; 4, the maximum value 72 of the voltage VT applied between members 24 and 26 is less than the potential breakdown level 74 between these members 24 and 26. A resistor R2 is also provided to limit the magnitude of the current flowing through the trigger portion of the circuit during arcing between members 24 and 26. The main power connections of the circuit to be controlled are made to conducting stems 26 and 34, generally as indi-' cated at 52 and 54, respectively.

A laser 50 and the related control circuitry (not shown) is provided to initiate operation of the vacuum gap device 10. To trigger the vacuum gap device a pulse laser beam, indicated generally at 56, of sufficient intensity, passes through the window 36 and is focused on the gas-saturated portion 28 of the trigger electrode 24. The energy ofthis laser beam 56 is sufficient to heat the gas-saturated portion 28 of trigger electrode 24 to the point to cause some of the absorbed gas, such as hydrogen, to be liberated from portion 28. Portion 28 is heated by laserbeam 56 and the gas is released in a periodof time much less than the period of the power line frequency. When portion 28 is sufficiently heated, a gas discharge will occur between the trigger electrode 24 and adjacent main electrode 20 supported by conducting stem'26. This will cause a low-power arc to be formed between trigger electrode 24 and main electrode 20. Electrodes 20 and 24 are constructed so that 'the loop of current flowing through a low-power arc and electrodes 20 and 24 causes a magnetic force on the low-power are. This magnetic force produced by the current loop, through the low-power arc, drives the low-power arc towards the interelectrode region 40 between the main electrbdes 20 and 22. The introduction of the low-power are into the region 40 indicates a power are 38 acrossthe main electrodes 20 and 22. When the vacuum gap device 10 is operated on an alternating current power line having a frequency of Hz or, 60 Hz,the time duration of the laser beamnecessary to liberate the absorbed gas from portion 28isvery short compared to the period of the power frequency. The main power are should be extinguished when the main power are current reaches a current zero during the alternating currentwave. The-laser 50 which initiates operation of the vacuum gap device 10 and any related control circuitry can be completely electrically isolated from the vacuum gap 10-Since a portion 32of insulating member 30 is of a smaller diameter than the inner-diameter of the conducting stem 26 metal vapor and particles from the power are 38 are less likely to be deposited on the outer surface 58 of portion 32. The window member 36 is recessed instem 34 so that its inner surface 34 is shielded from most of the particles and vapors expelled from are 38, and surface '35 is less likely to be coated with a metallic film, which would reduce the transparency of window 36.

Referring now to FIG. 5, there is shown a portion of the vacuum gap device [0 illustrating another embodiment of the invention. A gas-saturated metal piece 60 is attached to the main arching electrode 20. Note that a portion 62 of the inner diameter of main electrode 20 is beveled so that the gas-saturated metal part 60 is in position to be activated by a laser beam 56 projected through the stem 34 of the opposite electrode. The trigger electrode 24 extends through insulating member 30 and extends past the inner end 64 of insulating member 30. A voltage 70 or 72, less than the breakdown voltage 74 is applied between the trigger electrode 24 and the associated main electrode 20. As the laser beam 56 is focused on the gas-saturated metal piece 60, sufficient gas will be liberated from metal piece 60 to cause a low-power are 66 to form between the trigger electrode 24 and the main electrode 20. As illustrated by the arrow 68, the low-power are 66 is magnetically forced enters the main electrode area 40, a high-power are forms between electrodes 20 and 22.

Referring now toFlG. 6, there is shown a trigger electrode configuration similar to that shown in FIG. 5 except that the gas-saturated metalpiece 70 is attached to the trigger electrode 24 flush with insulating portion 30. As shown in FIG. 6, the trigger electrode 24 extends beyond the inner end of the insulating member 30, and passes through the gas-saturated 'metal disc 70. During operation of the vacuum gap device 10, the laser beam 56 is focused on the gas-saturated metal disc 70, so as to heat the disc 70 until the absorbed gas is released, thus causing a low-power are 66 to form between the trigger electrode 24 and the associated main electrode 20. The low-power are 66 is driven in the direction. as indicated'by arrows 68, due to the current flowing through the circuit comprising electrodes 20 and 24, into the main interelectrode area 40. The lowpower are 66 entering the interelectrode area 40 initiand forming a conducting path betweeen the main electrode 20 and the trigger electrode 24.

The power of the laser beam 56 needed to initiate the trigger discharge by the gas-saturated piece 28, or can be calculated as follows. Consider the gassaturated metal piece 28, 60 or 70 being an infinite plane. The temperature rise of the surface of metal piece 28, 60 or 70 as it being radiated by the laser beam 56 is given by the following formula:

where P radiation power density (watts/cm K thermal conductivity (cal/secK) 7 k thermal diffusivity t time (sec) J 4.2 joules/cal If the gas-saturated piece 28, 60 or 70 is made of titanium saturated with hydrogen, a large portion of the absorbed hydrogen will be evolved when the surface is heated to 1200C. Assuming that 50 percent of the laser beam energy is absorbed by the gas-saturated piece 28, 60, or 70, a pulsed ruby laser having a beam power density of 10 MW/cm and a pulse time duration of 10 nanoseconds 10 X l-'-seconds) would be sufficient to heat the titanium surface to 1200C. The time required to heat the surface of the gas saturated portion 28, 60 or 70, if this portion 28, 60 or 70 is fabricated from titanium (Ti), to 1200C can be calculated as follows:

Using a laser having'a power density of 10 X 10 watts/cm and assuming that 50 percent of the laser beam energy is absorbed by piece 28, 60 or 70.

' P 0.5 X 10 X 10 watts/cm 5 X watts/cm K varies in the range from 0.0425 to 0.0187 cal/secl( for T;

K 0.2 for Ti J i 4.2 joules/cal q A T l200C C (ambient) 11s0c' Rearranging Equation 1 to solve for time (I) gives:

substituting in the above values gives:

t= A/O.2 [(1.18 X 10 X 4.2 X 4 X 10 ]/[(lO X for K 0.04 cal/sec K 2 X 10' sec. 6.2 N sec for a value of K 0.019 cal/sec.K I

t= 1.3 X 10' sec.

As can be seen, this time is very much less than the period for a 50 Hz or 60 Hz wave.

The energy requirement of the pulse laser per area .per pulse, for K 0.04 cal/secl( is:

what lower power density. could be employedfln the example given, zirconium-aluminum alloy could be used instead of titanium for the gas-absorbed metal piece 28, 60 or 70. The advantage of using zirconiumaluminum is that it has a faster pumping speed for hydrogen. This would help to ensure that-the power are is extinguished when the power are current goes through a natural current zero.

The present invention has several advantages over prior art two electrode, laserinitiated, vacuum gap devices, as shown, for example, in U.S. Pat. No. 3,295,011 issued Dec. 27, 1966 to S. Barbini. For example, the three electrode system has a more controlled and faster breakdown voltage. In the present invention, the breakdown mechanism is different from prior art devices, in that the laser beam impinges upon a gas-saturated material causing a small plasma to form between the trigger electrode 24 and the main electrode 20. The plasma is then forced into main interelectrode region 40, causing a breakdown-between the alone produces triggering of the electric arc, while in.

the disclosed embodiment, the laser beam heats the gas-saturated material 28, 60 or to release a gas, and this gas is then ionized by an existing electric field between the trigger electrode 24 and the associated main electrode 20. The ionized gas conducts current and starts a low power are between electrodes 20 and 24.

I claim:

l. A triggered vacuum gap device comprising:

an evacuated envelope;

a first main arcing electrode disposed within said evacuated envelope;

a second main arcing electrode disposed within said evacuated envelope facing said first main arcing electrode, and spaced therefrom to form a first arcing gap therebetween;

a trigger electrode in close proximity to said second main electrode but electrically insulated therefrom by an open vacuum space, to form a second arcing gap therebetween;

a voltage source connected between said trigger electrode and said second main'electrode to form an electric field therebetween which has a value lower than the breakdown potential between said trigger electrode and said second main electrode;

a gas-saturated metal piece located near said trigger electrode;

laser means for heating said gas-saturated metal piece to release a gas in the second arcing region between said trigger electrode and said second main electrode to initiate a low-power arc therebetween; and,

magnetic means comprising said second main arcing electrode and said trigger electrode disposed with respect to said second main arcing electrode to form a current loop through said low-power are causing a magnetic force on said low-power arc for moving said low-power are into the first arcing gap region between said first main contact and said second main contact to initiate a main power are therebetween.

2. The apparatus as claimed in claim 1 wherein said triggering voltage source comprises a direct current supply and a current limiting resistor.

33. A triggered vacuum gap device as claimed in claim 1 wherein said trigger voltage source comprises an alternating current voltage supply, a voltage clipping device to limit peak value of said alternating voltage supply and a current limiting means.

4. A triggered vacuum gap device as claimed in claim 1 wherein a hollow conducting tube supports said first main arcing electrode, a transparent window supported in said hollow conducting tube, a laser beam projected from said laser means passing through said transparent window and said hollow conducting tube to impinge on said gas-saturated piece causing a gas to be released and initiating arcing between said trigger electrode and said second main arcing electrode.

prising asecond hollow conducting tube supporting said second main arcingelectro de, a portion of said trigger'electrode passing internal of said hollow conducting tube, insulating means for insulating said trigger electrode from said second hollow conducting tube, and said gas-saturated piece being attached to the end of said trigger electrode disposed within said hollow conducting tube.

6. A triggered vacuum gap as claimed in claim wherein a portion of said insulating means in close proximity to said gas-saturated piece is of a smaller diameter than said hollow conducting tube to form a gap between a portion of said insulating means and the inner diameter of said second hollow conducting tube so that arc-generated metallic vapors and particles cannot condense on the solid dielectric between said trigger electrode and said second hollow conducting tube to cause a shorting path therebetween.

7. A triggered gap vacuum device as claimed in claim 1 wherein a bevel is formed on the inner diameter portion of said second main arcing electrode, said gassaturated piece being mounted around said beveled portion of said second main arcing electrode so that when said laser means heats said gas-saturated piece, a

low-power arc is formed between said beveled portion of said second main arcing electrode and said trigger electrode.

8. A triggered vacuum 'gap as claimed in claim 1,

comprising: insulating means for supporting said trigger electrode, said gas-saturated piece comprises a metal disc that is saturated with hydrogen, said metal disc mounted at the end of said insulating means, said trigger electrode extending from said insulating means and passing through said gas-saturated metallic disc.

9. A triggered vacuum gap as claimed in claim 8 wherein said insulating means in the vicinity of said gassaturated metallic disc is undercut to form a gap between said insulating means and said second main arcing electrode so that arc-generated metallic vapors and particles cannot coat the surface and form a shorting pass between said trigger electrode and said second main arcing electrode.

10.- A triggered vacuum gap device comprising:

an evacuated envelope;

a first main arc-ing electrode disposed within said evacuated envelope;

a first hollow support being formed from a conducting material for supporting said first main arcing electrode;

a second main arcing electrode disposed within said evacuated envelope facing said first main arcing electrode and spaced therefrom to form first arcing gap therebetween;

a second hollow support being formed from a conducting material for supporting said second main arcing electrode;

a trigger electrode disposed within said second hollow support but electrically insulated therefrom to form a second arcing gap therebetween;

a voltage source connected between said trigger electrode and said second main arcing electrode to form an electric field therebetween which has a value lower than the breakdown potential between said trigger electrode and said second main electrode;

a gas-saturated metal piece located in close proximity to said trigger electrode;

laser means for heating said gas-saturated metal piece to release a gas in the second arcing gap between said trigger electrode and said second hollow support to initiate a low power arc therebetween; and,

said second hollow support and said trigger electrode being formed so that the magnetic field generated by the current flow when the low power arc is initiated forces-the low power arc into the first arcing gap between said first main arcing electrode and said second main arcing electrode to initiate a main power are therebetween.

11. A trigger vacuum gap device as claimed in claim 10 wherein:

said first hollow support comprises a hollow metallic tube extending through said evacuated envelope;

said laser means is disposed external to said evacuated envelope and directs a laser beam into said evacuated envelope through saidfirst hollow support; and including,

a transparent member disposed within said hollowsurface of said hollow metallic tube. 

1. A triggered vacuum gap device comprising: an evacuated envelope; a first main arcing electrode disposed within said evacuated envelope; a second main arcing electrode disposed within said evacuated envelope facing said first main arcing electrode, and spaced therefrom to form a first arcing gap therebetween; a trigger electrode in close proximity to said second main electrode but electrically insulated therefrom by an open vacuum space, to form a second arcing gap therebetween; a voltage source connected between said trigger electrode and said second main electrode to form an electric field therebetween which has a value lower than the breakdown potential between said trigger electrode and said second main electrode; a gas-saturated metal piece located near said trigger electrode; laser means for heating said gas-saturated metal piece to release a gas in the second arcing region between said trigger electrode and said second main electrode to initiate a lowpower arc therebetween; and, magnetic means comprising said second main arcing electrode and said trigger electrode disposed with respect to said second main arcing electrode to form a current loop through said lowpower arc causing a magnetic force on said low-power arc for moving said low-power arc into the first arcing gap region between said first main contact and said second main contact to initiate a main power arc therebetween.
 2. The apparatus as claimed in claim 1 wherein said triggering voltage source comprises a direct current supply and a current limiting resistor.
 3. A triggered vacuum gap device as claimed in claim 1 wherein said trigger voltage source comprises an alternating current voltage supply, a voltage clipping device to limit peak value of said alternating voltage supply and a current limiting means.
 4. A triggered vacuum gap device as claimed in claim 1 wherein a hollow conducting tube supports said first main arcing electrode, a transparent window supported in said hollow conducting tube, a laser beam projected from said laser means passing through said transparent window and said hollow conducting tube to impinge on said gas-saturated piece causing a gas to be released and initiating arcing between said trigger electrode and said second main arcing electrode.
 5. A triggered vacuum gap as claimed in claim 4 comprising a second hollow conducting tube supporting said second main arcing electrode, a portion of said trigger electrode passing internal of said hollow conducting tube, insulating means for insulating said trigger electrode from said second hollow conducting tube, and said gas-saturated piece being attached to the end of said trigger electrode disposed within said hollow conducting tube.
 6. A triggered vacuum gap as claimed in claim 5 wherein a portion of said insulating means in close proximity to said gas-saturated piece is of a smaller diameter than said hollow conducting tube to form a gap between a portion of said insulating means and the inner diameter of said second hollow conducting tube so that arc-generated metallic vapors and particles cannot condense on the solid dielectric between said trigger electrode and said second hollow conducting tube to cause a shorting path therebetween.
 7. A triggered gap vacuum device as claimed in claim 1 wherein a bevel is formed on the inner diameter portion of said second main arcing electrode, said gas-saturated piece being mounted around said beveled portion of said second main arcing electrode so that when said laser means heats said gas-saturated piece, a low-power arc is formed between said beveled portion of said second main arcing electrode and said trigger electrode.
 8. A triggered vacuum gap as claimed in claim 1, comprising: insulating means for supporting said trigger electrode, said gas-saturated piece comprises a metal disc that is saturated with hydrogen, said metal disc mounted at the end of said insulating means, said trigger electrode extending from said insulating means and passing through said gas-saturated metallic disc.
 9. A triggered vacuum gap as claimed in claim 8 wherein said insulating means in the vicinity of said gas-saturated metallic disc is undercut to form a gap between said insulating means and said second main arcing electrode so that arc-generated metallic vapors and particles cannot coat the surface and form a shorting pass between said trigger electrode and said second main arcing electrode.
 10. A triggered vacuum gap device comprising: an evacuated envelope; a first main arcing electrode disposed within said evacuated envelope; a first hollow support being formed from a conducting material for supporting said first main arcing electrode; a second main arcing electrode disposed within said evacuated envelope facing said first main arcing electrode and spaced therefrom to form first arcing gap therebetween; a second hollow support being formed from a conducting material for supporting said second main arcing electrode; a trigger electrode disposed within said second hollow support but electrically insulated therefrom to form a second arcing gap therebetween; a voltage source connected between said trigger electrode and said second main arcing electrode to form an electric field therebetween which has a value lower than the breakdown potential between said trigger electrode and said second main electrode; a gas-saturated metal piece located in close proximity to said trigger electrode; laser means for heating said gas-saturated metal piece to release a gas in the second arcing gap between said trigger electrode and said second hollow support to initiate a low power arc therebetween; and, said second hollow support and said trigger electrode being formed so that the magnetic field generated by the current flow when the low poWer arc is initiated forces the low power arc into the first arcing gap between said first main arcing electrode and said second main arcing electrode to initiate a main power arc therebetween.
 11. A trigger vacuum gap device as claimed in claim 10 wherein: said first hollow support comprises a hollow metallic tube extending through said evacuated envelope; said laser means is disposed external to said evacuated envelope and directs a laser beam into said evacuated envelope through said first hollow support; and including, a transparent member disposed within said hollow metallic tube through which the laser beam can pass and forming a vacuum tight seal with the inner surface of said hollow metallic tube. 